Toxin-induced activation of Rho GTP-binding protein increases Bcl-2 expression and influences mitochondrial homeostasis

Occurrence and genetic association of selected virulence factors in clinical Escherichia coli isolates

Occurrence of cnf1+ E. coli pathogenic strains among extraintestinal E. coli isolates was evaluated to explain an impact of cytotoxic necrotizing factor type 1 (CNF1) in human infections. A total of 120 E. coli isolates were characterized for presence of virulence factors cnf1- and pap--specific sequences by PCR, and the production of alpha-hemolysin using blood agar-plate test. Different association patterns among the detected virulence factors were obtained by comparison of various groups of clinical E. coli isolates. These differences probably reflect a potential impact of CNF1 in the colonization of vaginal environment.

Rho protein inactivation induced apoptosis of cultured human endothelial cells

Small GTP-binding Rho GTPases regulate important signaling pathways in endothelial cells, but little is known about their role in endothelial cell apoptosis. Clostridial cytotoxins specifically inactivate GTPases by glucosylation [Clostridium difficile toxin B-10463 (TcdB-10463), C. difficile toxin B-1470 (TcdB-1470)] or ADP ribosylation (C. botulinum C3 toxin). Exposure of human umbilical cord vein endothelial cells (HUVEC) to TcdB-10463, which inhibits RhoA/Rac1/Cdc42, or to C3 toxin, which inhibits RhoA, -B, -C, resulted in apoptosis, whereas inactivation of Rac1/Cdc42 with TcdB-1470 was without effect, suggesting that Rho inhibition was responsible for endothelial apoptosis. Disruption of endothelial microfilaments as well as inhibition of p160ROCK did not induce endothelial apoptosis. Exposure to TcdB-10463 resulted in activation of caspase-9 and -3 but not caspase-8 in HUVEC. Moreover, Rho inhibition reduced expression of antiapoptotic Bcl-2 and Mcl-1 and increased proapoptotic Bid but had no effect on Bax or FLIP protein levels. Caspase-3 activity and apoptosis induced by TcdB-10463 were abolished by cAMP elevation. In summary, inhibition of Rho in endothelial cells activates caspase-9- and -3-dependent apoptosis, which can be antagonized by cAMP elevation.

Mechanisms of NOS2 regulation by Rho GTPase signaling in airway epithelial cells

The aberrant dysregulation of the inducible form of nitric oxide synthase (NOS2) is thought to play a role in many inflammatory disorders including cystic fibrosis (CF). The complex regulation of NOS2 expression is the subject of intense investigation, and one intriguing regulatory pathway known to influence NOS2 expression is the Rho GTPase cascade. We examined NOS2 regulation in response to inflammatory cytokines in a human alveolar epithelial cell line treated with inhibitors of different upstream and downstream components of the Rho GTPase pathway to better define potential signaling mechanisms. Statin-mediated 3-hydroxy-3-methylglutaryl-CoA reductase inhibition increased cytokine-dependent activation of the NOS2 promoter, reversible by the addition of geranylgeranyl pyrphosphate. However, inhibition of Rho-associated kinase (ROCK) with Y-27632 resulted in a decrease in NOS2 promoter activity, yet an increase in NOS2 mRNA and protein levels. Our results suggest that prenylation events influence NOS2 promoter activity independently of the Rho GTPase pathway and that Rho GTPase signaling mediated through ROCK suppresses NOS2 production downstream of promoter function at the message and protein level.

How bacteria could cause cancer: one step at a time

Helicobacter pylori highlighted the potential for bacteria to cause cancer. It is becoming clear that chronic infection with other bacteria, notably Salmonella typhi, can also facilitate tumour development. Infections caused by several bacteria (e.g. Bartonella spp., Lawsonia intracellularis and Citrobacter rodentium) can induce cellular proliferation that can be reversed by antibiotic treatment. Other chronic bacterial infections have the effect of blocking apoptosis. However, the underlying cellular mechanisms are far from clear. Conversely, several bacterial toxins interfere with cellular signalling mechanisms in a way that is characteristic of tumour promoters. These include Pasteurella multocida toxin, which uniquely acts as a mitogen, and Escherichia coli cytotoxic necrotizing factor, which activates Rho family signalling. This leads to activation of COX2, which is involved in several stages of tumour development, including inhibition of apoptosis. Such toxins could provide valuable models for bacterial involvement in cancer, but more significantly they could play a direct role in cancer causation and progression.

Rac1 prevents cisplatin-induced apoptosis through down-regulation of p38 activation in NIH3T3 cells

In this study, the role of V12-Rac1 in the cisplatin-induced apoptosis was investigated. Cisplatin-induced apoptosis is associated with cytochrome c release, which can be inhibited by V12-Rac1 expression. The analysis of mitogen-activated protein kinase activity indicated that V12-Rac1 expression led to a decrease in p38 activity after exposure to cisplatin but not c-jun N-terminal kinase and extracellular signal-regulated kinase. Using pharmacological inhibitors, it was found that only p38 is a critical mediator in the cisplatin-induced apoptosis of NIH3T3 cells. This suggests that V12-Rac1 can stimulate the anti-apoptotic signaling pathway in response to cisplatin, and that decreased p38 activity caused by V12-Rac1 expression in cisplatin-treated NIH3T3 cells is crucial for V12-Rac1-dependent cell survival.

Inhibition of protein geranylgeranylation and RhoA/RhoA kinase pathway induces apoptosis in human endothelial cells

Geranylgeranylation of RhoA small G-protein is essential for its localization to cell membranes and for its biological functions. Many RhoA effects are mediated by its downstream effector RhoA kinase. The role of protein geranylgeranylation and the RhoA pathway in the regulation of endothelial cell survival has not been elucidated. The hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitor lovastatin depletes cellular pools of geranylgeranyl pyrophosphate and farnesol pyrophosphate and thereby inhibits both geranylgeranylation and farnesylation. Human umbilical vein endothelial cells (HUVECs) were exposed to lovastatin (3 microm-30 microm) for 48 h, and cell death was quantitatively determined by cytoplasmic histone-associated DNA fragments as well as caspase-3 activity. The assays showed that lovastatin caused a dose-dependent endothelial cell death. The addition of geranylgeraniol, which restores geranylgeranylation, rescued HUVEC from apoptosis. The geranylgeranyltransferase inhibitor GGTI-298, but not the farnesyltransferase inhibitor FTI-277, induced apoptosis in HUVEC. Cell death was also induced by a blockade of RhoA function by exoenzyme C3. In addition, treatment of HUVEC with the RhoA kinase inhibitors Y-27632 and HA-1077 caused dose-dependent cell death. Y-27632 did not inhibit other well known survival pathways, such as NF-kappa B, ERK, and phosphatidylinositol 3-kinase/Akt. However, there was an increase in p53 protein level concomitant with Y-27632-induced cell death. Unlike the apoptosis induced by TNF-alpha, which occurs only with inhibition of new protein synthesis, apoptosis induced by inhibitors of HMG-CoA reductase, geranylgeranyltransferase, or RhoA kinase was blocked by cycloheximide. Our data indicate that inhibition of protein geranylgeranylation and RhoA pathways induce apoptosis in HUVEC and that induction of p53 or other proapoptotic proteins is required for this process.

The cytotoxic necrotizing factor 1 (CNF1) from Escherichia coli

The cytotoxic necrotizing factor 1, from uropathogenic Escherichia coli, is the paradigm of Rho-GTPases-activating bacterial toxins. CNF1 is a MW 108kDa A-B protein toxin divided into three domains which are implicated in the three steps of the intoxication process. The N-terminal domain contains the cell receptor function and binds with high affinity to a cell receptor not yet identified. Binding of the toxin is followed by its internalization by endocytosis and its transport into late endosomes. The middle toxin domain contains two hydophobic helices which allow translocation of the toxin across the membrane upon acidification in late endosomes. Finally the carboxy-terminal domain of CNF1 is an enzyme which deamidates Rho-GTP-binding proteins (Rho, Rac and Cdc42) glutamine 63 (for Rho) or glutamine 61 (for Rac and Cdc42). Deamidation of glutamine 63/61 blocks the intrinsic or the GTPase activating protein (GAP)-induced hydrolysis of GTP leading to the permanent activation of the GTPase. Activation of Rho GTPases by CNF1 induces a profound reorganization of the cell actin cytoskeleton. By its properties on Rho GTPases CNF1 is to date an invaluable tool for cell biology studies.

An essential role for Rac/Cdc42 GTPases in cerebellar granule neuron survival

Rho family GTPases are critical molecular switches that regulate the actin cytoskeleton and cell function. In the current study, we investigated the involvement of Rho GTPases in regulating neuronal survival using primary cerebellar granule neurons. Clostridium difficile toxin B, a specific inhibitor of Rho, Rac, and Cdc42, induced apoptosis of granule neurons characterized by c-Jun phosphorylation, caspase-3 activation, and nuclear condensation. Serum and depolarization-dependent survival signals could not compensate for the loss of GTPase function. Unlike trophic factor withdrawal, toxin B did not affect the antiapoptotic kinase Akt or its target glycogen synthase kinase-3beta. The proapoptotic effects of toxin B were mimicked by Clostridium sordellii lethal toxin, a selective inhibitor of Rac/Cdc42. Although Rac/Cdc42 GTPase inhibition led to F-actin disruption, direct cytoskeletal disassembly with Clostridium botulinum C2 toxin was insufficient to induce c-Jun phosphorylation or apoptosis. Granule neurons expressed high basal JNK and low p38 mitogen-activated protein kinase activities that were unaffected by toxin B. However, pyridyl imidazole inhibitors of JNK/p38 attenuated c-Jun phosphorylation. Moreover, both pyridyl imidazoles and adenoviral dominant-negative c-Jun attenuated apoptosis, suggesting that JNK/c-Jun signaling was required for cell death. The results indicate that Rac/Cdc42 GTPases, in addition to trophic factors, are critical for survival of cerebellar granule neurons.

Cooperative control of Akt phosphorylation, bcl-2 expression, and apoptosis by cytoskeletal microfilaments and microtubules in capillary endothelial cells

Capillary endothelial cells can be switched between growth and apoptosis by modulating their shape with the use of micropatterned adhesive islands. The present study was carried out to examine whether cytoskeletal filaments contribute to this response. Disruption of microfilaments or microtubules with the use of cytochalasin D or nocodazole, respectively, led to levels of apoptosis in capillary cells equivalent to that previously demonstrated by inducing cell rounding with the use of micropatterned culture surfaces containing small (<20 microm in diameter) circular adhesive islands coated with fibronectin. Simultaneous disruption of microfilaments and microtubules led to more pronounced cell rounding and to enhanced levels of apoptosis approaching that observed during anoikis in fully detached (suspended) cells, indicating that these two cytoskeletal filament systems can cooperate to promote cell survival. Western blot analysis revealed that the protein kinase Akt, which is known to be critical for control of cell survival became dephosphorylated during cell rounding induced by disruption of the cytoskeleton, and that this was accompanied by a decrease in bcl-2 expression as well as a subsequent increase in caspase activation. This ability of the cytoskeleton to control capillary endothelial cell survival may be important for understanding the relationship among extracellular matrix turnover, cell shape changes, and apoptosis during angiogenesis inhibition.

Leukotoxicity of pyoverdin, production of reactive oxygen species, and effect of UV radiation

Pyoverdin was purified by solvent extraction, gel filtration, and ionic exchange chromatography. Assays of cytotoxic of pyoverdin were done with human leukocytes and macrophages from the peritoneum of mice. Both cell quantities showed a significant reduction. Death was followed by lysis in a dose-dependent form. The mechanism of action of pyoverdin involved the stimulation of reactive oxygen species (ROS) measured by Nitroblue Tetrazolium (NBT) reaction and chemiluminescence (CL). UV radiation at 368 nm increased the leukotoxicity; expositions of 5 min were enough to photostimulate the effect of pyoverdin on cellular oxydative metabolism, which increased between 35.4 and 53.2%. Genestein, an inhibitor of tyrosine kinases, counteracted the ROS stimuli of pyoverdin, suggesting endocytic mechanism of action for this pigment. The little chloroquine interference on oxydative stress indicated that intraphagosomal pH and the stimuli of reactive nitrogen intermediaries (RNI) seem to be of less importance than ROS in pyoverdin action on leukocytes.

Opposite effects of microtubule-stabilizing and microtubule-destabilizing drugs on biogenesis of mitochondria in mammalian cells

Distribution of mitochondria as well as other intracellular organelles in mammalian cells is regulated by interphase microtubules. Here, we demonstrate a role of microtubules in the mitochondrial biogenesis using various microtubule-active drugs and human osteosarcoma cell line 143B cells and rat liver-derived RL-34 cells. Depolymerization of microtubules by nocodazole or colchicine, as well as 2-methoxyestradiol, a natural estrogen metabolite, arrested asynchronously cultured cells in G(2)/M phase of cell cycle and at the same time inhibited the mitochondrial mass increase and mtDNA replication. These drugs also inhibited the mitochondrial mass increase in the cells that were synchronized in cell cycle, which should occur during G(1) to G(2) phase progression in normal conditions. However, stabilization of microtubules by taxol did not affect the proliferation of mitochondria during the cell cycle, yet a prolonged incubation of cells with taxol induced an abnormal accumulation of mitochondria in cells arrested in G(2)/M phase of cell cycle. Taxol-induced accumulation of mitochondria was not only demonstrated by mitochondria-specific fluorescent dyes but also evidenced by the examination of cells transfected with yellow fluorescent protein fused with mitochondrial targeting sequence from subunit VIII of human cytochrome c oxidase (pEYFP) and by enhanced mtDNA replication. Two subpopulations of mitochondria were detected in taxol-treated cells: mitochondria with high Delta(psi)(m), detectable either by Mito Tracker Red CMXRos or by Green FM, and those with low Delta(psi)(m), detectable only by Green FM. However, taxol-induced increases in the mitochondrial mass and in the level of acetylated (alpha)-tubulin were abrogated by a co-treatment with taxol and nocodazole or taxol and colchicine. These data strongly suggest that interphase microtubules may be essential for the regulation of mitochondrial biogenesis in mammalian cells.

Evidence for Rho protein regulation of renal tubular epithelial cell function

BACKGROUND

Rho proteins are small guanine 5'-triphosphate (GTP)-binding proteins felt to be important regulators of several aspects of cell function, including the organization of the actin cytoskeleton. The effects of Rho proteins on the regulation of renal tubular epithelial cell function are not known.

METHODS

Selected bacterial toxins that inhibit Rho protein function were used to examine the effect of Rho in cultured renal tubular epithelial cells.

RESULTS

Clostridium difficile toxin A significantly and dose dependently inhibited LLC-PK(1) cell (3)H-thymidine uptake and healing of small wounds made in confluent monolayers, and it induced apoptosis. A second Clostridium difficile toxin (toxin B) that acted via a different receptor also impaired LLC-PK(1) thymidine uptake and wound healing, and it induced apoptosis. A third bacterial toxin, C3 toxin from Clostridium botulinum, also impaired LLC-PK(1) thymidine uptake and stimulated apoptosis in LLC-PK(1) cells. Since Rho inhibition disrupted organization of the actin cytoskeleton, we examined the effects of another agent that disrupted the actin cytoskeleton (cytochalasin D) and found significant dose-dependent effects that impaired LLC-PK1 thymidine uptake and wound healing and that induced apoptosis. The effects of toxin A and cytochalasin D to induce apoptosis were not associated with significant changes in expression of Bcl-2, BAD, or BAK proteins and were significantly attenuated by a pancaspase inhibitor.

CONCLUSIONS

Our results suggest that Rho proteins are important endogenous regulators of several aspects of renal tubular epithelial cell function, including proliferation, migration, and apoptosis. Further studies are needed to clarify the cellular mechanisms of Rho regulation of renal epithelial cell function.

Prevalence of virulence genes and clonality in Escherichia coli strains that cause bacteremia in cancer patients

Phenotypic analysis of Escherichia coli strains causing bacteremia in cancer patients suggests that they possess specific virulence properties. To investigate this hypothesis, we compared the frequency of the virulence-related genes cnf1, cnf2, papC, hlyC, and iut in 155 E. coli strains isolated from hospitalized cancer patients with epidemiologically unrelated cases of bacteremia to their frequency in 70 E. coli strains isolated from the feces of healthy unrelated volunteers. Of the blood isolates, 24, 37, and 26% were positive for cnf1, papC, and hlyC, respectively, versus only 6, 17, and 6% of the fecal isolates (P < 0.05 in all instances). By contrast, 47% of both isolates carried the iut gene. The patients' clinical characteristics did not significantly influence these frequencies. The presence on various pathogenicity islands (PAIs) of a combination of the cnf1, papC, and hlyC genes on the chromosome was strongly suggested by Southern blotting of pulsed-field gel electrophoresis (PFGE) patterns with specific DNA probes. The phylogenetic relatedness among 60 strains carrying three, two, one, or no virulence genes and 6 ECOR strains included as references was determined by neighbor joining, the unweighted pair-group method with arithmetic mean, and Wagner analysis of the randomly amplified polymorphic DNA (RAPD) patterns generated by 11 primers. Identification of a major cluster including 96.4% of the strains carrying the cnf1, papC, and hlyC genes and ECOR subgroup B2 strains suggested that the virulent E. coli strains causing bacteremia in cancer patients are closely related to ECOR B2 strains. The presence in the E. coli population surveyed of a strong linkage disequilibrium, and especially of a highly significant correlation between PFGE and RAPD genetic distances, confirms that clonal propagation has a major impact on the E. coli population structure. Nevertheless, low bootstrap values in the phylogenetic tree suggested that frequent genetic exchange inhibits the individualization of discrete genetic lineages, which are stable on an evolutionary scale.

Clostridium difficile toxin A causes early damage to mitochondria in cultured cells

BACKGROUND & AIMS

The mechanism by which Clostridium difficile toxin A causes actin depolymerization and cell rounding involves toxin internalization and subsequent monoglucosylation of the Rho family of proteins. This study explored toxin internalization and effects on mitochondrial function before cell rounding.

METHODS

Chinese hamster ovary (CHO) cells were exposed to toxin A, and mitochondrial localization was assayed by confocal microscopy. Mitochondrial function was measured by adenosine triphosphate (ATP) concentration, mitochondrial permeability, and leakage of cytochrome c.

RESULTS

Confocal microscopy showed toxin A colocalization with the mitochondrial protein GRP 75 at 5 minutes after toxin exposure. Between 5 and 15 minutes, toxin A caused an 80% diminution in cellular ATP levels; cell rounding and Rho glucosylation commenced between 15 and 30 minutes. Toxin A also resulted in reduction of mitochondrial membrane potential and a 2-3-fold increase in reactive oxygen radicals. Preincubation of CHO cells with the antioxidants butylated hydroxyanisole or butylated hydroxytoluene blocked the toxin A-induced increase in oxygen radicals and diminished cell rounding. Western blot analysis of toxin A-exposed isolated mitochondria showed a direct effect of toxin A on leakage of cytochrome c.

CONCLUSIONS

The results show that extensive mitochondrial damage occurs within 15 minutes in CHO cells exposed to toxin A. Diminished ATP concentrations and increased oxygen radicals are likely to contribute to cytotoxicity from this bacterial toxin.

Rho family proteins modulate rapid apoptosis induced by cytotoxic T lymphocytes and Fas

Little is known about the role of Rho proteins in apoptosis produced by stimuli evolved specifically to produce apoptosis, such as granzymes from cytotoxic T lymphocytes (CTLs) and Fas. Here we demonstrate that all three Rho family members are involved in CTL- and Fas-induced killing. Dominant-negative mutants of each Rho family member and Clostridium difficile toxin B, an inhibitor of all family members, strongly inhibited the susceptibility of cells to CTL- and Fas-induced apoptosis. Fas-induced caspase-3 activation was inhibited by C. difficile toxin. Activated mutants of each GTPase increased susceptibility to apoptosis, and activation of Cdc42 increased within 5 min of Fas stimulation. In contrast, during the time required for CTL and Fas killing, no apoptosis was produced by dominant-negative or activated mutants or by C. difficile toxin alone. Inhibition of actin polymerization using latrunculin A reduced the ability of constitutively active GTPase mutants to stimulate apoptosis and blocked Fas-induced activation of caspase-3. Furthermore, the ability of Rac to enhance apoptosis was decreased by point mutations reported to block Rac induction of actin polymerization. Rho family proteins may regulate apoptosis through their effects on the actin cytoskeleton.

Ras-GTPase activating protein inhibition specifically induces apoptosis of tumour cells

Oncogenes and tumour suppressor genes control the balance between apoptotic death and anti-apoptotic survival signals determining whether a cell proliferates or dies. Through which effectors might oncoproteins generate sensitivity to apoptosis remains to be determined. Ras GTPase activating protein (Ras-GAP) is a key element in the Ras signalling pathway, being both a negative regulator and possibly an effector of Ras. Ras-GAP acts as a regulator of transcription, and possibly connects Ras to stress-activated protein kinases. A role for Ras-GAP in cell survival has been suspected from the study of knock-out mouse embryos. In search for selective killing of tumour cells, we asked whether Ras-GAP inhibition by other means would lead to apoptosis in established cell lines. We injected a monoclonal antibody directed against the SH3 domain of Ras-GAP (mAb200) that has been shown to block Ras-GAP downstream signalling into various human normal and tumour cell lines. We show that inhibition of Ras-GAP induces apoptosis specifically in tumour, but not in normal cells, therefore pointing at a specific role for Ras-GAP in tumour cell survival. MAb200-induced apoptosis is largely prevented by coinjection of activated RhoA or Cdc42 proteins, by injection of a constitutively activated mutant of phosphoinositide 3-OH kinase (PI3-K), but not by injection of v-Raf. These results show that targeting of Ras-GAP could represent a novel anticancer approach.

Inosine monophosphate dehydrogenase: A molecular switch integrating pleiotropic GTP-dependent beta-cell functions

Studies of pancreatic islet function in the pathogenesis of type 2 diabetes mellitus have tended to focus on the short-term control of insulin secretion. However, the long-term control of beta-cell mass is also relevant to diabetes, since this parameter is reduced substantially even in non-insulin-dependent diabetes in humans. In animal models of type 2 diabetes, the normal balance between beta-cell proliferation and programmed cell death is perturbed. We take the perspective in this overview that inosine monophosphate dehydrogenase (IMPDH; EC 1.1.1. 205) may represent a previously neglected molecular integrator or sensor that exerts both functional (secretory) and anatomical (proliferative) effects within beta-cells. These properties reflect the fact that IMPDH is a rate-limiting enzyme in the new synthesis of the purine guanosine triphosphate (GTP), which modulates both exocytotic insulin secretion and DNA synthesis, as well as a number of other critical cellular functions within the beta-cell. Alterations in the expression or activity of IMPDH may be central to beta-cell replication, cell cycle progression, differentiation, and maintenance of adequate islet mass, effects that are probably mediated both by GTP directly, and indirectly via low molecular mass GTPases. If GTP becomes depleted, a hierarchy of beta-cell functions becomes progressively paralyzed, until eventually the effete cell is removed via apoptosis.

The 5' region of cnf1 harbours a translational regulatory mechanism for CNF1 synthesis and encodes the cell-binding domain of the toxin

The Escherichia coli cytotoxic necrotizing factor 1 (CNF1) is organized into three functional domains: the N-terminal part containing the cell-binding domain, a putative central membrane-spanning region, and a C-terminal catalytic region. On the basis of competition assays between CNF1 and GST-recombinant proteins containing different N-terminal fragments, and point mutations, we restricted the binding region to the first 190 amino acids. Hydrophilic amino acids 53-75 are strictly necessary to cell receptor recognition. Using different cnf1-lacZ translational fusions, we demonstrated that the mRNA corresponding to the first 48 codons of cnf1 is involved in the translational regulation of CNF1 synthesis. This regulation consists of both a positive and a negative control. The positive control is exerted by codons 6-20, including a putative downstream box that enhances the translational expression of cnf1. The negative control depends on codons 45-48. In this region, an anti-Shine-Dalgarno sequence, highly homologous to the core of the internal complementary sequence already reported for growth rate-regulated metabolic genes, has been detected. To some extent, the inner structural organization of CNF1 would thus suggest the compiling of several functions in a single mRNA protein system.

Clostridium difficile toxin B induces apoptosis in intestinal cultured cells

Toxigenic strains of the anaerobic bacterium Clostridium difficile produce at least two large, single-chain protein exotoxins involved in the pathogenesis of antibiotic-associated diarrhea and colitis. Toxin A (CdA) is a cytotoxic enterotoxin, while toxin B (CdB) is a more potent cytotoxin lacking enterotoxic activity. This study dealt with CdB, providing the first evidence that intestinal cells exposed to this toxin exhibit typical features of apoptosis in that a significant proportion of the treated cells displayed nuclear fragmentation and chromatin condensation. In keeping with ultrastructural data, CdB-treated cells showed the typical flow cytometric hallmark of apoptosis consisting of a distinct sub-G1 peak. The CdB-induced apoptotic response was dose and time dependent and not simply due to the actin-disrupting effect of the toxin or to the subsequent impairment of cell anchorage. Rather, the inhibition of proteins belonging to the Rho family due to CdB seems to play a role in the induction of apoptosis in intestinal cells. The origin of cells and the growth rate may also be cofactors relevant to such a response.